Many human diseases, collectively called ?ciliopathies?, are caused by the dysregulation of cilia and ciliary signaling and often present with neurological defects. Development pathways such as Hedgehog and WNT are modulated by the cilium and are deeply involved in neural development. Further, recent evidence suggests that cilia and ciliary signaling respond to chemical and mechanical cues as environmental ?sensors?. Here, we propose to study a unique function of cilia in zebrafish spinal cord development. The Isacoff laboratory recently discovered that VALopA, a non-visual G-protein linked opsin, expressed in the zebrafish spinal cord and suppressed neural activity and locomotive movement when activated. Recent results have showed the VALopA localizes to cilia in the spinal cord, likely in motor neurons. These results suggest that motor neurons in the spinal cord may integrate light signals through VALopA and modulate neural and circuit development. I will utilize motor neuron specific Gal4 fish lines and VALopA CRISPR knock-out fish lines (currently available in the lab), live in vivo imaging with biosensors, superresolution in vivo imaging, pharmacological analysis, and electrophysiology to probe if and how light can function through cilia in spinal cord neurons to influence neural and circuit development. My specific aims are as follows: Aim 1: Direct sensory signaling through cilia in motor neurons. Aim 2: Ciliary GPCR signaling: gating neuronal activity. Aim 3: Ciliary GPCR signaling: gating the ciliary beat. The phenomenon that a non-visual opsin in cilia could turn motor neurons into light sensitive cells, would represent a new mechanism for neural circuit modulation and development. This could also suggest a broader function of cilia in the nervous system: to serve as environmental sensors that mediate circuit development and tune circuit activity. These results could also inform future studies on the neurological defects caused by cilia dysregulation.